Power semiconductor devices, such as power diodes, or power MOSFETs, are designed to withstand high blocking voltages. Those power devices include a pn-junction that is formed between a p-doped semiconductor region and an n-doped semiconductor region. The component blocks (is switched off) when the pn-junction is reverse biased. In this case a depletion region (also referred to as space charge region) propagates in the p-doped and n-doped regions. Usually, one of these semiconductor regions is more lightly doped than the other one of these semiconductor regions, so that the depletion region mainly extends in the more lightly doped region, which mainly supports the voltage applied across the pn-junction. The semiconductor region supporting the blocking voltage is referred to as base region in a diode, and is referred to as drift region in a MOSFET.
The ability of a pn-junction to support high voltages is limited by the avalanche breakdown phenomenon. As a voltage applied across a pn-junction increases, an electric field in those semiconductor regions that form the pn-junction increases. The electric field results in an acceleration of mobile charge carriers that are present in the semiconductor region. An avalanche breakdown occurs when, due to the electric field, the charge carriers are accelerated such that they create electron-hole pairs by impact ionization. Charge carriers created by impact ionization create new charge carriers, so that there is a multiplication effect. At the onset of an avalanche breakdown, a significant current flows across the pn-junction in the reverse direction. The voltage at which the avalanche breakdown sets in is referred to as breakdown voltage.
The electric field at which the avalanche breakdown sets in is referred to as critical electric field (Ecrit). The absolute value of the critical electric field is mainly dependent on the type of semiconductor material used for forming the pn-junction, and is weakly dependent on the doping concentration of the more lightly doped semiconductor region.
The critical electric field is a theoretical value that is defined for a semiconductor region that has an infinite size in directions perpendicular to field strength vectors of the electric field. Power semiconductor components, however, have semiconductor bodies of finite size that are terminated by edge surfaces in lateral directions. In vertical power semiconductor devices, which are semiconductor devices in which the pn-junction mainly extends in a horizontal plane of the semiconductor body, the pn-junction usually does not extend to the edge surface of the semiconductor body but is distant to the edge surface of the semiconductor body in a lateral direction. In this case, a semiconductor region (edge region) of the semiconductor body adjoining the pn junction in the lateral direction also has to withstand the blocking voltage.
In so-called superjunction devices, the pn-junction has a relatively large area. These devices include compensation regions doped complementary to the drift region and adjoining the drift region. pn-junctions between the compensation regions and the drift region form a part of the overall pn-junction of the semiconductor device. The compensation regions serve to compensate dopant carriers in the drift region when the pn-junction is reverse biased. This compensation effect allows the implementation of a drift region with a higher doping concentration than a conventional (non-superjunction) device, resulting in a lower on-resistance at a given voltage blocking capability.
When the pn-junction is forward biased, minority charge carriers emitted from the compensation regions into the drift region and majority charge carriers in the drift region form a charge-carrier plasma. When the pn-junction is reverse biased, this charge-carrier plasma has to be removed from the drift region before the semiconductor device blocks. A high quantity of charge carriers stored in the edge region when the pn-junction is forward biased may result in a reduced voltage blocking capability of the edge region, and may result in a destruction of the semiconductor device.